Survey of Pest resistance to Bt crops

Jonas Kathage has posted a very approachable survey of Bt resistance issues and strategies at Biology Fortified, Inc. Jonas closes with the following discussion of both economics and resistance management strategies:

Meanwhile, entomologists are working on improving their incomplete understanding the complex mechanisms involved in resistance evolution. Recently published research suggests that pyramiding might not work as well in delaying resistance as previously thought. In the laboratory, scientists selected cotton bollworm (Helicoverpa zea) for resistance against Cry1Ac. They exposed the resistant insects and a susceptible control group to Bt cotton expressing Cry1Ac/Cry2Ab and found that the group resistant to Cry1Ac exhibited a much higher survival rate than the control group, violating the assumption of redundant killing that is crucial to this strategy. So far, despite multiple reported instances of resistant insects, large-scale failure of Bt crops due to evolved resistance has not occurred, but it may come sooner than expected.

Should refuge requirements be expanded?

This research finding is bad news because the potential of pyramided Bt crops might be lower than believed. (Actually, some scientists have been positively surprised at the long delays observed in resistance development.) Let’s assume the results also apply to other Bt pyramids and insect species (there is evidence to the contrary). What should be made of such a scenario? Should larger refuge areas be required?

Before answering that question, it must be recognized that the sustainable application of a particular technology is not a primary goal of farming. A much more important goal is efficiency. Efficiency means getting the most output (e.g. food) from a set of scarce inputs (natural resources, labor, capital). The technologies transforming inputs into outputs, be they biological, chemical, or mechanical, are valuable only insofar as they contribute towards efficiency.

When deciding whether to expand refuge requirements, policymakers must take into account that there is a tradeoff between the size of the refuge area and productivity. If refuge area increases, more plants will get damaged by pests and hence reduce effective yield. The crucial question is whether the benefits of delaying resistance outweigh the costs of these yield losses and other potential drawbacks of refuges such as the need for additional land, sprays, separation costs, and sowing and harvest times. Costs of monitoring compliance with refuge requirements must also be considered, while pyramiding will incur more R&D expenditures. (In some developing countries with larger monitoring costs, refuge requirements may be less efficient also because of natural refuge in small-scale cropping systems.) The point here is not to question whether the optimal refuge requirement is 0%, 20% or 40%, but to realize that there are costs that have to be weighed against benefits. It is possible that an arms race based on adding more Bt genes is more efficient than slowing resistance development by expanding mandated refuges.

Besides Bt crops, there is a host of other pest management options including chemical control, biological control and cultural control such as ploughing and crop rotation. Like Bt, they all have their particular drawbacks, be it risk of resistance development, low effectiveness, or environmental and economic cost. The most efficient pest management strategy depends on local context, but will involve multiple instruments. For breeders, genes producing insect toxins, whether introduced using conventional or GM techniques, are not the only route towards pest protection. There are exciting possibilities on the horizon, including transgenic plants that emit volatile organic compounds to repel herbivores or attract their natural enemies. The use of nano-silica that kill pests by purely physical means are just one example of potential applications of nanotechnology in pest management. New approaches will have benefits and costs to be assessed against existing alternatives. As of today, there are no magic bullets protecting crops from pests. But there are excellent reasons that we should keep looking for them. Bt will not be the end of the road.

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